1. Technical Field of the Invention
This invention relates to a suspension system for window blinds, shutters or the like, and to control apparatus adapted for use therewith.
Common manually operated blind or curtain suspension systems, which may be either the standard manually operable type or the corded manually operable type, can comprise the security of the dwelling place during periods of absence, and it is therefore desirable to provide a blind/curtain suspension system which lends itself readily to automatic operation. Such a system would not only alleviate the problem of security but would also be of benefit to the disabled, blind, elderly or infirm occupants.
2. Description of the Prior Art
Exiting automatic blind/curtain suspension systems generally include a support carrying at least one suspension device arranged for movement relative to the support towards and away from a stop to open and close the blind/curtain, and an electric motor coupled to the suspension device and operable to cause movement of the suspension device relative to the support. Such suspension systems have hitherto operated in one of two ways. In the first, motion of the suspension device or devices relative to the support is arrested at either end of its travel by means of dead stops, i.e. fixed stops against which the leading suspension device impacts at one or other extreme of its travel. The effect of this is that the motor stalls, which leads almost instantaneously to a very large increase in motor current, known as "overcurrent". Overcurrent detectors may be provided to switch power to the motor off should the current to the motor exceed a predetermined value, but by the time overcurrent is detected, the motor has already stalled and the motor and the remainder of the system suffered the mechanical stresses induced.
Apart from the mechanical stresses mentioned above, a further problem arises with the use of dead stops. When the leading suspension device impacts the stop, the motor will continue to drive for a very short period of time until the torque to which the suspension device is tightened against the stop reaches the stall torque rating of the motor. At this point, the motor stalls. If sufficient friction is present in the system, e.g. where the suspension device is carried by a nut traversing a threaded, driven shaft, releasing the suspension device from abutment with the stop requires the motor to exert a torque greater than and opposite to that which tightened the suspension device against the stop in the first place. Inertia present in the motor and drive mechanism when the motor is brought to a halt will also be taken up in tightening the suspension device against the stop. This, together with the fact that static friction instead of dynamic friction must be overcome, means that there is a good chance that the suspension device will jam against the stop and need to be released by hand.
As an alternative to the use of dead stops, some available blind/curtain suspension systems utilize limit switches which are actuated by the passing suspension device. The use of limit switches contributes significantly to the cost of the system, requiring additional wiring and additional control hardware, for example to allow the motor to be reversed once it has tripped a limit switch, but advanced no further until it has been reversed. This again contributes to the cost, making the use of limit switches a relatively expensive option.